Display device for helmet-mounted display

ABSTRACT

A display device for a helmet-mounted visual system including an intensified image-taking device and a visor to combine the intensified images and an ambient scene before the pilot&#39;s eyes. The display device also includes an optical prism whose optical aberrations (image distortions, astigmatism) are capable of compensating for the aberrations created by the visor of the helmet, on the images given to the pilot.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An object of the present invention is a device enabling the manufactureof light-weight, ergonomic helmet-mounted visual display systems, inwhich a luminous image is seen in a state where it is superimposed onthe view of the outside scene.

2. Background of the Invention

This type of helmet-mounted visual system is used in aeronautics, theobserver being the pilot of an airline or helicopter.

In particular, warplanes or combat helicopters require the presentationof piloting and firing control information as well as intensified imagesof the surroundings of the airline or helicopter. All this informationmay be presented on a head-up visor present in all warplanes and incertain helicopters.

The drawback of this type of display is that it can present an imageonly in a limited field that is always centered on the axis of theaircraft while the pilot may have to take sightings at a distance fromthe axis of the aircraft. This is why, at present, display devices areintegrated into the pilot's helmet, with the pilot carrying the field ofdisplay with him. Integrated devices of this kind enable the pilot tokeep autonomous systems, especially intensified image-taking systems,with him. These may comprise in particular an image-taking objectivecoupled with a light intensifier, a relaying optical system for theprojection, on the visor of the helmet, of an image that is sent backtowards the pilot's eyes in being superimposed on an ambient scene.

Despite these advantages, the presentation of the image on a visorsuffers from defects inherent in its use in projection on a tiltedvisor.

These defects are in particular of two kinds: the first kind relates toimage distortion. Indeed, owing to major variations of incidence on thevisor due to its inclination as a function of the field observed, theimage projected on a visor is distorted (this is often called the secondtype of off-center distortion).

The second kind of defect relates to the astigmatism introduced into theimage. The tilting of the visor has, for a given field, an influence onthe quality of the image within one and the same field: the radius ofcurvature seen along a given plane differs from that observed along aplane orthogonal to the first plane. The rays, with respect to these twoplanes, therefore get focused at different places.

To compensate for these optical defects, certain solutions have beenconsidered. In particular, it is possible to use a toroidal visor tocompensate for distortion and astigmatism.

However, the designing of a toroidal visor remains difficult toimplement.

Another approach consists of the introduction, after the image-takingobjective, of an optical aberration correction device using a CCDcircuit and then a cathode-ray tube to recreate a corrected image. Adevice of this kind increases the weight of a pilot's helmet andgenerally calls for a high voltage supply that is incompatible with anindependent system capable of being carried with the pilot when heejects.

SUMMARY OF THE INVENTION

To overcome these different drawbacks, the invention proposes a displaydevice for a helmet-mounted visual system using at least one opticalprism having optical aberrations that are capable of compensating forthe distortion and astigmatism type of aberrations created by the visorin the images.

Indeed, in a prism, at refraction during the passage between the air andthe middle of the prism, the non-linearity of the variations of incidentangles and refracted angles induces an image deformation of the sametype as the one shown in FIG. 1. FIG. 1 pertains to a matrix of dots ofan undistorted image. FIG. 1b pertains to a matrix of image dotsdistorted by the visor. Furthermore, the optical prism has an invarianceby translation along a particular plane. Its properties of refractiontherefore differ within one and the same field between this plane andthe plane that is orthogonal to it. This phenomenon is similar toastigmatism.

More specifically, an object of the invention is a display device forhelmet-mounted visual systems comprising:

an intensified image-taking device delivering a first optical beam;

a visor;

means for the superimposition, before an observer, of an outside sceneand the first optical beam delivered by the intensified image-takingdevice through an relaying optical system;

characterized in that the relaying optical system comprises at least oneoptical prism to compensate for the distortion and astigmatismintroduced by the visor on said first optical beam.

The display device for a helmet-mounted visual system may advantageouslyfurthermore comprise an image generator,emitting a second optical beamthat carries information, and a mixer so as to superimpose the firstoptical beam and the second optical beam, at the relaying opticalsystem.

According to one variant of the invention, the means used to superimposean outside scene and the intensified images taken are integrated intothe visor.

According to another variant of the invention, the means used tosuperimpose an outside scene and the intensified images taken comprisean independent combiner made of glass.

Indeed, the invention can also be applied in the case of a combiner notintegrated into the visor which may have a curved reflecting surface asshown in FIG. 2, this reflecting surface creating the same type ofdistortion is as a visor. This type of combiner comprises a curvedreflecting surface Sc with a center of curvature C, coupled with acomplementary prism PC to direct the optical beam of intensified imagesor synthetic images L towards the pilot's eye 11.

To improve the performance characteristics of the device withoutencumbering it, the optical prism may comprise means to make the opticalbeams perform at least one outward journey and one return journey insaid optical prism.

The optical prism used in the device according to the invention mayadvantageously comprise a curved face so as to approach the visor effectfor improved compensation of the optical aberrations introduced by thevisor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood more clearly and other advantages shallappear from the following description given on a non-restrictive basiswith reference to the appended figures, of which:

FIGS. 1a and 1 b illustrate the image distortions created by thecurvature of a helmet visor;

FIG. 2 illustrates an exemplary combiner used in a display deviceaccording to the invention;

FIG. 3 illustrates a first exemplary display device for a helmet-mountedvisual system according to the invention comprising a prism working intransmission mode;

FIG. 4 illustrates a second exemplary display device for ahelmet-mounted visual system according to the invention comprising aprism working in reflection mode;

FIG. 5 illustrates a third exemplary display device for a helmet-mountedvisual system according to the invention comprising an image generator;

FIG. 6 illustrates an exemplary prism used in the invention;

FIG. 7 illustrates an exemplary helmet using a device according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a first variant of the invention, the display device hasonly one prism working in transmission mode, the means for thesuperimposing of images and of the outside scene being integrated intothe visor.

As shown in FIG. 3, the helmet-mounted visual system comprises a visor 1that has locally undergone a dielectric or holographic type surfacetreatment 01 in order to reflect a light beam that is a carrier ofintensified images L1, while at the same time enabling the transmissionof an outside scene. The light beam L1 is created from a device 2comprising an image-taking objective 3 and a light intensifier 4. Thelight intensifier comprises a photocathode at input and a cathode-rayscreen at output. The photons received on the image-taking objective areconverted into electrons by the photocathode. The light intensifieraccelerates and multiplies these electrons to finally form anintensified image on its cathode ray screen. This intensified image issent through an relaying optical system 5 consisting in particular of acertain number of lenses 6 and transmission means 8 to direct theintensified image towards the visor. The relaying optical system alsocomprises the optical prism 7. The pilot's eye 11 thus observes the beamof intensified images Li and the outside scene.

According to a second variant of the invention, the display devicecomprises a prism working in reflection mode. One of the majorconstraints of helmet-mounted visual systems is the fact that thecurrently used systems are bulky. Any gain in mass obtained in each ofthe elements of the helmet-mounted visual system is important. This iswhy, it may be particularly worthwhile to duplicate the passage of alight beam in the medium used to correct optical aberrations, in thiscase the optical prism. This double passage makes it possible to reducethe weight of the optical prism used by a factor of 2. This amounts to amajor advantage in the case of prisms made of glass having a highoptical index but also high density. The display device of this variantshown in FIG. 4 is very similar to the device shown in FIG. 3. In thisvariant, the transmission means are integrated into the optical prismwhich works in reflection mode.

According to a third variant of the invention, the helmet-mounted visualsystem furthermore comprises an image generator enabling the informationto be superimposed on the images perceived by the pilot. This variant ofthe invention is shown in FIG. 5.

In addition to the image-taking device 2, the helmet-mounted visualsystem comprises an image generator 9 which, for example, may be asymbol generator generating a light beam L2. It also comprises a mixingoptical system 10 receiving, firstly, the intensified images borne bythe light beam L1 and, secondly, the images created by the imagegenerator and borne by the light beam L2.

According to other variants of the invention, the optical beams L1 andL2 may perform more than one outward and return journey in the opticalprism. This may be sought notably for an optical prism made of plastic,which is advantageous in terms of weight as compared with glass but hasa refraction index (close to 1.5) smaller than that of the types ofglass used (close to 1.8) and therefore introduces fewer opticalcorrections.

For this purpose, the optical prism may be provided with a reflectivetreatment on one part only of one of its faces and also at least on onepart of another one of its faces as illustrated in FIG. 6.

The optical prism used in the invention may also comprise, on one of itsfaces, a treatment that is selective in terms of angle of incidence.

FIG. 7 illustrates a helmet fitted out with the display device describedhere above with reference to FIG. 5. The helmet is a binocular versionused to project information on both eyes of the pilot.

The pilot's helmet has two first lateral housings for the image-takingdevices 2, two second lateral housings for the image generators 9 whichcan be located above said first lateral housings and two mixers 10. Thehelmet also has a set of two relaying optical systems 5 and two prisms7, at the output of which the generated images and the intensifiedpictures taken may be superimposed on the outside scene before thepilot's eyes 11.

In FIG. 7, only the left-hand display device integrated into the helmetis shown. The same device (not shown) is integrated into the right-handpart of the helmet.

What is claimed is:
 1. Helmet-mounted visual system comprising: anintensified image-taking device configured to deliver a light beam; avisor; means for superimposing, before an observer, an outside sceneviewed through the visor and the light beam delivered by the intensifiedimage-taking device; a relaying optical system located between theintensified image-taking device and the visor, wherein the relayingoptical system comprises at least one optical prism to compensate for asecond type of off-center distortion and astigmatism introduced by thevisor on the light beam delivered by the intensified image-takingdevice; and wherein the visor singly covers both eyes of the observer.2. Helmet-mounted visual system according to claim 1, furthercomprising: an image generator configured to emit an informationcarrying light beam; and a mixer configured to superimpose the lightbeam delivered by the intensified image-taking device and theinformation-carrying light beam at the relaying optical system. 3.Helmet-mounted visual system according to claim 1, wherein theintensified image-taking device comprises an image-taking objective, anda light intensifier.
 4. Helmet-mounted visual system according to claim1, wherein the visor comprises a semi-reflective treated surfaceconfigured to superimpose, before the observer, the outside scene andthe light beam delivered by the intensified image-taking device or theoutside scene and the light beam delivered by the intensifiedimage-taking device superimposed with an information-carrying lightbeam.
 5. Helmet-mounted visual system according to claim 1, wherein thevisor comprises a holographic treated surface configured to superimpose,before the observer, the outside scene and the light beam delivered bythe intensified image-taking device or the outside scene and the lightbeam delivered by the intensified image-taking device superimposed withan information-carrying light beam mixer.
 6. Helmet-mounted visualsystem according to claim 1, further comprising a combiner made of glassconfigured to superimpose, before the observer, the outside scene andthe light beam delivered by the intensified image-taking device or theoutside scene and the light beam delivered by the intensifiedimage-taking device superimposed with an information-carrying lightbeam.
 7. Helmet-mounted visual system according to claim 1, wherein theoptical prism comprises means for making the light beam delivered by theintensified image-taking device or the light beam delivered by theintensified image-taking device superimposed with aninformation-carrying light beam perform at least one to-and-fro journeyin said optical prism.
 8. Helmet-mounted visual system according toclaim 6, wherein the optical prism comprises two faces that are at leastpartially reflective for the light beam delivered by the intensifiedimage-taking device or for the light beam delivered by the intensifiedimage-taking device superimposed with an information-carrying lightbeam.
 9. Helmet-mounted visual system according to claim 6, wherein theoptical prism comprises a face with a treatment the treated face beingselectively transparent or reflective based on angle of incidence of thelight beam delivered by the intensified image-taking device and theinformation-carrying light beam.
 10. Helmet-mounted visual systemaccording to claim 1, wherein the optical prism comprises a face that iscurved to increase compensation for the second type of off-centerdistortion and the astigmatism introduced by the visor on the light beamdelivered by the intensified image-taking device or on the light beamdelivered by the intensified image-taking device superimposed with aninformation-carrying light beam.